The present invention is directed generally to treating the surface of metals for increased hardening and corrosion resistance, and more particularly to laser phase transformation and ion implantation in metals.
Lasers have been used for surface modification of ferrous metals. The high-energy flux associated with high-power laser beams can produce rapid localized heating at the surface of the metal. Depending upon the optical absorption coefficient of the metal at the laser illumination wavelength, most of the laser energy can be absorbed in a thin layer at the surface of the metal target. Use of a pulsed laser with pulses at a duration of a few microseconds or less produces very high surface temperatures, while the bulk of the material remains cool. When the laser pulse is extinguished, the heat deposited at the metal surface is rapidly self-quenched by conduction and diffusion into the cooler bulk of the metal. This rapid heating and cooling at the material surface causes transformation hardening, in which the surface material is transformed into phases that are harder and more wear resistant than the bulk of the metal. However, this process, as heretofore proposed is not well suited for use in conjunction with thin metal structures. The absence of material bulk does not provide cooling at a sufficient rate to generate the desired phase transformations.
Additionally, prior carbon deposition or carburizing techniques utilize controlled atmospheres, and polluting, hazardous or toxic gases. For instance, vapor deposition usually involves breaking down CH4 or other hydrocarbon gases in a vacuum or low pressure atmosphere.
A method and apparatus are provided for treating the surface of a metal body through phase transformation, ion implantation and/or diffusion and to form new phases of metal materials. The method and apparatus have been shown to be particularly useful to improve the hardness and corrosion resistance of ferrous and non-ferrous metals. Generally, the method comprises irradiating a portion of the metal body with a laser, and directing a stream of gas onto the same portion of the metal body simultaneously with and preferably for a duration after the laser is turned off. Preferably, the laser is a carbon dioxide laser operated in a pulsed mode to control heating of the metal. The gas is preferably carbon dioxide to quickly cool the metal when the laser is off, and to provide carbon atoms for deposition onto the metal body. The entire process may be carried out in an environment at atmospheric pressure obviating the need for a vacuum chamber or pressure controlled furnace or similar apparatus. After treatment, the hardness and corrosion resistance of at least a ferrous metal body are dramatically increased. Advantageously, both sides of a thin metal body, such as a metal tube, may be simultaneously treated. Further, a new, highly oxygenated, hard and extremely corrosion resistant metal phase may be created.
It is therefore a general object of the present invention to provide an economical process for modifying the surface characteristics of metals, particularly ferrous metals, to increase surface hardness and/or corrosion resistance. Another object of the present invention is to provide a process of the described character that operates at atmospheric conditions. A further object of the invention is to provide a process of the described character that will simultaneously treat both surfaces of a thin metal object such as a metal tube. A still further object of the invention is to provide a process of the described character which uses a common, non-polluting gas. Another object of the invention is to form new metal phases. A still further object of the invention is to introduce atoms into metal substrates by diffusion and by ion implantation with the rapid and relatively simple method of the invention.